The invention relates to color field sequence detection and, in particular, to a digital circuit for detecting a specific color field in the color field sequence of a color television signal, for NTSC, PAL or PAL-M color television standards, wherein the usual phase locked loop is eliminated.
In the field of color television, the detection and identification of a specific color field in the color field sequence of an NTSC, PAL, or PAL-M color television standard, is a well known process that is required when performing such processes as color field editing, color framing of videotape recorders, time base correction of color signals, etc. For example, a time base corrector must convert any off-tape field to the field type dictated by the station reference. Likewise, the proper color field must be known when performing an edit process to prevent the generation of undesirable picture shifts.
In turn, color field detection requires the identification of a phase relationship between the color subcarrier signal and the horizontal sync. This relationship for the color fields of the color field sequence in an incoming video reference signal, differs in accordance with the standard.
Because of the television raster scan system of interlace, adjacent fields are offset vertically by one-half scan line. In addition, the color encoding structure is not identical on corresponding lines of different fields. Thus, for example, in NTSC there are two unique color frames each having two interlaced fields. The fields are normally indicated as fields 1 and 2 of color frame A and fields 3 and 4 of color frame B. Fields 1 and 3 differ in that the encoding chroma subcarrier phase will be 180.degree. offset for a given line on field 1 compared to the corresponding line on field 3. The same is true when fields 2 and 4 are compared. In PAL, the sync-to-subcarrier relationship and the V axis phase gives four unique frames (eight unique fields).
Typically, detection of the phase relationship of previous mention is performed in an analog domain by using color burst as a reference to synthesize phase coherent subcarrier, and comparing the latter signal to the edge of horizontal sync during the proper line of each video frame. When the comparison meets the selected specification for subcarrier-to-horizontal sync (ScH) phase, detection of the specified color field, generally color field 1, can be made consistently.
Analog color field sequence detectors presently found, for example, in videotape recorders, editors, etc., require significant analog circuitry. This, in turn, maximizes the susceptibility of the detector to the noise, drift, etc., problems commonly associated with analog circuits. In addition, most of the circuitry must be duplicated for each standard which is handled by the apparatus, thus increasing the components, the complexity, the space required and the costs.
In the analog system, a phase locked loop is employed to continuously re-generate the color subcarrier in a phase locked condition over successive intervals of horizontal lines. The synthesized phase coherent subcarrier then is compared to the edge of horizontal sync during the proper line of each video field. As is well known, phase locked loops are susceptible to the problems of noise, drift, added component count and difficulty in board layout.
The present invention circumvents the disadvantages of the above analog detector circuits while providing the inherent advantages of an all-digital circuit which is fast enough to make the subcarrier-to-sync comparison in the same horizontal line, thereby eliminating the need for a phase locked loop and its inherent disadvantages. To this end, in a preferred embodiment, a reference signal of subcarrier frequency is generated internally, starting with a known phase relationship at the leading edge of H sync. The phase of this reference signal then is compared to incoming burst of the same line when it occurs. Since burst occurs a relatively short time after H-sync, a free-running time base is sufficient to make an accurate phase comparison, and all phase locking circuitry is eliminated.
More particularly, an analog processor section includes a sync stripper and an odd/even field detector to provide composite sync and odd field signals, and a burst processor to provide a squared burst signal. A digital processor section includes a line selector circuit for determining a selected video line signal, and further includes a timing generator responsive to the selected line signal, the composite sync and an NTSC/PAL standard signal for internally generating the reference signal of subcarrier frequency, starting at the leading edge of composite sync. In addition, the timing generator provides a state clock signal, and one or more sample pulses for timing the subsequent phase comparison to be made by a burst phase comparator. The phase comparator also receives the squared burst and, under control of the sample pulses, compares its phase to that of the reference subcarrier signal, whereby the phase relationship may be determined. A state machine circuit receives the phase comparison results and, in response to the standard signal and the state clock, supplies a field reference signal which selectively becomes active to identify the presence of color field 1.
Thus, it may be seen that the reference signal generation, phase detection, sequence detection and timing generation all are performed in the digital domain with its inherent advantages of ease of board layout, lower component count, less board space requirements, adaptability to multiple color television standards and operating stability, as well as the elimination of the phase locked loop as previously mentioned. The invention further lends itself to gate array implementation.